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Related Concept Videos

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How big should the pilot study for my cluster randomised trial be?

Sandra M Eldridge1, Ceire E Costelloe2, Brennan C Kahan2

  • 1Centre for Primary Care and Public Health, Queen Mary University of London, London, UK s.eldridge@qmul.ac.uk.

Statistical Methods in Medical Research
|June 14, 2015
PubMed
Summary

External pilot studies for cluster randomized trials often lack sufficient sample size. This can lead to imprecise and biased estimates for the main trial's sample size calculation, particularly for the intra-cluster correlation coefficient.

Keywords:
cluster randomised trialintra-cluster correlationpilotsample size

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Area of Science:

  • Clinical Trials Methodology
  • Biostatistics
  • Health Services Research

Background:

  • Pilot studies are crucial for planning larger randomized controlled trials.
  • Cluster randomized trials (CRTs) require specific considerations for sample size due to correlated data within clusters.
  • External pilot studies aim to inform the design of the main trial, including sample size estimation.

Purpose of the Study:

  • To determine the necessary sample size for external pilot studies of cluster randomized trials.
  • To evaluate the precision of estimating key parameters for main trial sample size calculations.
  • To assess the reliability of estimating process measures like recruitment rates from pilot studies.

Main Methods:

  • Simulations were used to model the distribution of expected clusters for the main trial.
  • Assumptions included varying cluster sizes, intra-cluster correlation, and pilot study decisions.
  • Intra-cluster correlation values and pilot study sizes reflected common literature values.

Main Results:

  • Pilot studies often provide downwardly biased and imprecise estimates of the main trial's required sample size.
  • Sufficient precision for parameter estimation (e.g., intra-cluster correlation coefficient) requires large numbers of clusters and participants.
  • Pilot studies are frequently too small to reliably estimate recruitment or follow-up rates.

Conclusions:

  • External pilot studies for cluster randomized trials are typically underpowered for precise parameter estimation.
  • Inadequate pilot study size compromises the accuracy of sample size calculations for the main trial.
  • Recommendations emphasize the need for larger pilot studies to ensure reliable parameter and rate estimations.